Diesel combustion exhaust is a regulated emission. Diesel particulate matter (PM), is the particulate component of diesel exhaust, which includes diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, PMs can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometer range of 100 nanometers. Various technologies have been developed for identifying and filtering out exhaust PMs before the exhaust is released to the atmosphere.
As an example, soot sensors, also known as PM sensors, may be used in vehicles having internal combustion engines. A PM sensor may be located upstream and/or downstream of a diesel particulate filter (DPF), and may be used to sense PM loading on the filter and diagnose operation of the particulate filter. Typically, a resistive PM sensor may sense a soot level based on a correlation between a measured change in electrical conductivity (or resistivity) between a pair of electrodes placed on a planar substrate surface of the sensor with the amount of PM deposited between the measuring electrodes. Specifically, the measured conductivity provides a measure of soot accumulation because the PM is composed primarily of electrically conductive carbon soot, with a smaller fraction of lower conductivity components such as volatile organics and metal oxides (oil ash).
One example PM sensor design is shown by Roth et al. in U.S. Pat. No. 8,823,401B2. Therein, a pair of planar adjacently placed interdigitated electrodes, either placed with a gap between them or juxtaposed together, connected to a common voltage source are used to independently detect PMs in the exhaust. As the PMs deposit on the interdigitated electrode pair due to electrostatic attraction between the charged PMs and the electrodes, the output of the two independent PM sensors are further analyzed and compared using extensive algorithms to derive meaningful information about the amount of PMs in the exhaust.
However, the inventors herein have recognized potential issues with such an approach. The PM sensors described by Roth et al. may continue to have reduced sensitivity due to the poor electrostatic attraction experienced by the PMs located away from the sensor surface in the electric field generated by the electrode pair. While the strength of the electric field in the region between each planar interdigitated electrode pair is higher near the surface of the electrode pair, the electric field decays rapidly away from it. Additionally, the sensor output of Roth et al., requires analysis with extensive algorithms to derive meaningful information regarding PM in the exhaust, leading to extended processing times and undesired delays in data output and diagnostics.
The inventors have identified an approach to partly address these issues while improving the sensitivity of the PM sensors. In one example approach, PM sensor reliability may be improved by a method comprising of generating a first electric field via a planar interdigitated electrode pair and generating a second electric field via the planar interdigitated electrode pair and a second planar element parallel with the planar interdigitated electrode pair. As a result, the strength of the electric field generated in the region between the two interdigitated electrode pairs which is normal to the surface of the interdigitated electrode pairs, can be increased, thereby increasing the electrostatic attraction of the PMs and increasing sensitivity of the PM sensors.
As an example, the PM sensor assembly could comprise of a planar interdigitated electrode pair and a conducting plate which is held at a voltage bias compared to the electrode pair; in an alternate embodiment, the conducting plate could be replaced by a second planar interdigitated electrode pair, again held at a voltage bias with respect to the first interdigitated electrode pair, such that there is an additional electric field created normal to the surface of the PM sensors. The technical effect of using such a PM sensor assembly to detect exhaust soot is that, the additional electric field between the conducting plate (or the second planar interdigitated electrode pair) and the first planar interdigitated electrode pair increases the electrostatic attraction thereby increasing the amount of soot that gets deposited on the PM sensor, thereby improving sensitivity of the PM sensor assembly to the detection of soot. Further, by increasing the voltage bias, the electric field strength may be increased. In one example configuration, where the two PM sensors face each other, the sensitivity of each PM sensor in the assembly may be increased by increasing the voltage bias. By using the collective output of both the sensors, a more accurate measure of the exhaust soot load, and thereby the DPF soot load can be determined. In addition, the increased sensitivity of the PM sensor allows for the rapid detection of PMs leaking downstream of a degraded DPF. As such, this improves the efficiency of filter regeneration operations, and reduces the need for extensive algorithms. In addition, by enabling more accurate diagnosis of an exhaust DPF, exhaust emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.